A water pan is used in BBQ smokers to add moisture to the cooking chamber.
Some people think adding different liquids (beer, wine, juice, soda, etc.) will help with flavoring the meat but we don’t think it adds much if any. So don’t waste the good beverages… just use water. The BBQ flavor will come from your rubs and the smoke.
Water pans also help to maintain an even temperature in the cooker. By boiling at a constant 212 degrees the steam from the water pan will help to keep temperatures constant.
And finally, water pans can be used for indirect cooking. In BBQ smokers like a UDS or WSM the water pan creates a shield between the heat source and the meat. If this is the goal, a water pan can be filled with sand to be used as a diffuser.
Tip: Wrap your water pan in foil prior to using to make clean up easier.